1. Definitions
Following are art recognized terms which are used herein to describe physical characteristics and propagation characteristics of optical fibers. These terms are well known in the art and can be found in such texts and publications as Optical Fiber Telecommunications, edited by S. E. Miller et al., Academic Press, 1979 (especially Chapter 3); Optical Fibers for Transmission by J. E. Midwinter, John Wiley & Sons, 1979 (especially Chapters 5 and 6); Theory of Dielectric Waveguides by D. Marcuse, Academic Press, 1974; and my U.S. Pat. No. 4,715,679.
The present invention relates to optical fibers for communication purposes wherein the relative refractive index difference .DELTA. is much less than 1, the term .DELTA. being defined as ##EQU1##
Fibers having such small values of .DELTA. are called weakly guiding fibers; the propagation constants of their guided modes are represented by .beta. such that EQU n.sub.0 k&lt;.beta.&lt;n.sub.1 k (2)
where n.sub.1 is the peak refractive index of the core, n.sub.0 is the cladding index and k, the propagation constant of plane waves in vacuum, is equal to 2.pi./.lambda., .lambda. being the wavelength.
The normalized frequency V is a dimensionless number that determines the number of modes a fiber can support; it can be defined as follows: EQU V=(n.sub.1.sup.2 -n.sub.0.sup.2).sup.1/2 ka (3)
where a is the core radius. The term V.sub.c.sup.j is the normalized cutoff frequency of the jth mode, the term V.sub.c.sup.1 being the normalized cutoff frequency of the first higher order mode. The operating V-value is V.sub.o.
A more convenient way of representing the propagation constant is by the normalized propagation constant b, which is defined as ##EQU2## The normalized propagation constant b depends on the refractive index profile of the fiber and the normalized frequency V. An example of such behavior is shown for step index fibers in FIG. 3.3 of the publication Optical Fiber Telecommunications. In general, for more complicated refractive index profiles, such propagation curves of b vs. V are obtained by numerical calculations by computer modeling.
When more than one mode propagates in an optical fiber, the difference in delay times of the fastest and slowest propagating modes limits the bandwidth of the fiber. The normalized delay time of the jth mode is given by .vertline.d(Vb)/dVj.vertline..sub.j, where j represents the highest order mode that propagates with low loss and 0 represents the fundamental mode. The difference between the normalized delay times of the fundamental mode and the jth mode can be characterized by ##EQU3##
In single-mode waveguides the total dispersion is governed by the material dispersion D.sub.m and the waveguide dispersion D.sub.w. For a given fiber composition, the material dispersion varies as a function of wavelength. For example, the material dispersion versus wavelength curve passes through zero dispersion at a wavelength near 1280 nm for high silica content fibers. Single mode fibers can be designed which exhibit zero total dispersion at any wavelength in a range of wavelengths above that wavelength at which the material dispersion curve passes through zero dispersion. This can be achieved by balancing out material dispersion with waveguide dispersion at some specified wavelength which is selected because of low fiber attenuation and/or availability of light sources. A convenient quantity for analyzing the waveguide dispersion is Vd.sup.2 (Vb)/dV.sup.2, the normalized waveguide dispersion, which is related to waveguide dispersion D.sub.w as follows: ##EQU4## where c is the speed of light. A graph of normalized waveguide dispersion versus the ratio (V/V.sub.c.sup.1) enables one to compare the relative waveguide dispersions that can be obtained for different fiber core refractive index profiles.
2. Field of the Invention
The present invention relates to optical fibers for use in communication systems, and more particularly, to optical fibers that are characterized by high bandwidth, few mode operation at a predetermined band of wavelengths and by low dispersion, single-mode operation at longer wavelengths.
Multimode fibers are advantageous for certain applications such as local area networks since inexpensive connectors and sources can be employed. However, the bandwidth of a conventional multimode fiber is relatively low since the group delays of modes are different. A solution to this dilemma involves the utilization of a fiber that is designed such that only a few modes propagate, the normalized delay times of the propagating modes coinciding at or near the operating V-value V.sub.o. Also, the difference between the normalized delay times of the propagating modes caused by V-value deviation from V.sub.o should be as small as possible. Some few-mode fibers are designed to operate at a V-value where the j+1 mode is lossy. If the cutoff value of the j+1 mode is V.sub.c.sup.j+1, the fiber may operate at a V-value V.sub.o that is up to about 1.1 V.sub.c.sup.j+1. Thus, only the normalized delay times of the low loss propagating modes up to the jth mode are taken into consideration. As will be discussed below, this lossy j+1 mode is potentially troublesome. Since few-mode fibers have both relatively large information carrying capacities relative to conventional multimode fibers and have relatively large core diameters, as compared to single-mode fibers, they have been considered for use in local area networks.
A few-mode optical fiber would be especially advantageous if it were also capable of low dispersion single-mode operation. As used herein, the term "low dispersion" means a total dispersion of less than 5 ps/km-nm. Such a fiber would be useful in a number of systems applications. For example, a system could be operated at a first wavelength at which the fiber propagates a few modes in order to take advantage of inexpensive sources and connectors when bandwidth requirements are relatively low to moderate, i.e. greater than 1 GHz-km and preferably in the range of 2-4 GHz-km. Later, when bandwidth requirements increase, the system can be upgraded by employing terminal equipment which operates at a higher bit rate and by utilizing a source and detector that operate in the low dispersion, single-mode region of the fiber.
A few-mode/single-mode fiber would also be useful in a system in which data must be transmitted at a relatively high data rate in one direction but wherein relatively low to moderate data rate propagation is permissible in the opposite direction. A high performance laser might be employed to initiate the propagation of the high data rate signal at one end of the fiber. A less expensive source, operating at a shorter wavelength, could be employed to initiate the propagation of the few mode signal.
3. The Prior Art